This disclosure relates to apparatus and methods for applying thermal energy to work pieces, such as for bonding, sealing, cutting and the like of the work pieces. More particularly, the present disclosure relates to an apparatus and method for applying ultrasonic energy to work pieces such as disposable diapers, training pants, feminine care products, feminine care and incontinence garments, or the like.
Use of mechanical vibration produced at an ultrasonic frequency to weld thermoplastics, and to emboss and form plastics is a well-established industrial process. The physical principles underlying this technology have important relations to the disclosure described herein and therefore merit brief review and discussion.
To obtain significant vibrational motion, most ultrasonic systems are operated at one of their frequencies of resonance. Both the ultrasonic generator and the ultrasonic horn are designed to resonate at the same frequency, in which case the vibration produced by the generator is communicated to the horn. Because the horn is tuned to the same frequency as the generator, the horn expands and contracts along its length in concert with the imposed motion of the vibration generator.
The motion produced at the free face of the horn is then reciprocal, or back and forth in a surface perpendicular to the surface of the horn, with an amplitude determined by the electrical voltage applied to the crystals of the vibration generator. It is known to condition the vibrations produced by the generator before the vibrations are communicated to the horn, including incorporating amplification devices and phase change devices into the sequence of elements so used.
Bond strengths are a closely monitored quality target in, for example, personal care articles such as adult incontinence products. A particular bond of concern is the side seam bond in such articles. Side seam bonding has historically been plagued by periods of low average bond strengths and high bond strength variability. Solid modeling and lab testing has shown that the anvils used in current processes have resonant frequencies close to the side seam bonder's ultrasonic operating frequency. When the ultrasonic operating frequency is close to an anvil resonance frequency, the anvil vibrates in ways that generally are detrimental to bond strength (higher variability, lower bonding effectiveness), generator power requirements, generator operating stability, ultrasonic component reliability, and ultrasonic component longevity.